This application is based on application No. H11-114397 filed in Japan on Apr. 22, 1999, the entire content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an image-sensing apparatus, and in particular to an image-sensing apparatus incorporating a solid-state image-sensing device having a characteristic that an output voltage thereof varies natural-logarithmically in accordance with the amount of received light.
2. Description of the Prior Art
FIG. 1 shows a solid-state image-sensing element (hereafter referred to as a xe2x80x9cpixelxe2x80x9d) employed in a conventional solid-state image-sensing device having a characteristic that an output voltage thereof varies natural-logarithmically in accordance with the amount of received light. The pixel shown in FIG. 1 is composed of a photodiode PD that receives at its cathode a voltage Vdd1, an N-channel MOS transistor Tr1 that has its drain and gate connected to the anode of the photodiode PD and that receives at its source a voltage Vss1, an N-channel MOS transistor Tr2 that has its gate connected to the gate of the transistor Tr1 and that receives at its drain a voltage Vdd2, and a capacitor C that has one end connected to the source of the transistor Tr2 and that receives at the other end a voltage Vss2. Here, the voltage appearing at the node between the source of the transistor Tr2 and the capacitor C is used as the output voltage Vout. The voltages mentioned above fulfill the relations Vdd1 greater than Vss1 and Vdd2 greater than Vss2.
How this pixel works will be described briefly below. This pixel exploits the subthreshold characteristics that the transistor Tr1 exhibits when its gate-source voltage is lower than the threshold voltage. When light is shone on the photodiode PD and, as a result, a current Ip flows through the transistor Tr1, the gate voltage Vg of the transistor Tr1 varies natural-logarithmically in accordance with the current Ip. This gate voltage Vg causes a current to flow also through the transistor Tr2, and thus electric charge is accumulated in the capacitor C. As a result of this electric charge being accumulated in the capacitor C, the output voltage Vout appears, which is given as                     Vout        =                  Vss1          +                                    nkT              q                        ⁢                          ln              ⁡                              (                                                      q                    nkTC                                    ⁢                                      ∫                                          Ip                      ⁢                                              ⅆ                        t                                                                                            )                                                                        (        1        )            
where q represents the amount of electric charge carried by an electron, k represents the Boltzmann constant, n represents a constant determined according to the structure of the transistor, T represents the absolute temperature, and C represents the capacitance of the capacitor C.
A solid-state image-sensing device has a plurality of pixels, each having a characteristic as described above, arranged in an array, and is further provided with transfer means for transferring the output voltages from the individual pixels. In such a solid-state image-sensing device, the individual pixels have different sensitivities, and therefore, even if uniform light is shone on them, the voltages output from the individual pixels differ from one another. To overcome this problem, U.S. Pat. No. 5,289,286 proposes an image-sensing apparatus in which the differences in sensitivity among the individual pixels are compensated for.
The configuration of the image-sensing apparatus proposed in U.S. Pat. No. 5,289,286 mentioned above will be described briefly below with reference to FIG. 6, which shows a block diagram thereof. The image-sensing apparatus shown in FIG. 6 is composed of an image-sensing device (hereafter referred to as the xe2x80x9csensorxe2x80x9d) 1 that has a plurality of pixels each producing an output voltage varying natural-logarithmically in accordance with the amount of received light and that has transfer means for transferring the outputs from the individual pixels, an analog-to-digital converter (hereafter referred to as the xe2x80x9cA/D converterxe2x80x9d) 2 for performing analog-to-digital conversion on the voltage output from the sensor 1, a memory 3 for storing the digital value obtained as a result of conversion performed by the A/D converter 2, a correction calculation circuit (hereafter referred to as the xe2x80x9cCORxe2x80x9d) 4 for subtracting the shading data stored beforehand in the memory 3 from the image data fed from the A/D converter 2 during actual image shooting so as to correct the image data, and a device 5, such as a storage device or printer, that is fed with the image data corrected by the COR 4.
In this image-sensing apparatus configured as described above, first, uniform light is shone on the sensor 1, and, for each pixel, the digital data representing the output voltage obtained therefrom is stored in the memory 3. Then, during actual image shooting, the image data acquired is corrected on the basis of the above-mentioned digital data, called the shading data, that was stored in the memory 3 when uniform light was shone on the sensor 1, and the thus corrected image data is output to the device 5. By correcting the image data in this way, it is possible to correct the unevenness in sensitivity among the individual pixels of the sensor 1.
Suppose that an amount L of light is shone on a pixel having a sensitivity xcex1. Then, the output voltage Vout1, which varies in accordance with the amount L of light and the temperature T of the sensor 1, is given by Equation (2) below. On the other hand, when an amount L0 of uniform light is shone on the sensor 1 to store the shading data in the memory 3, the pixel having a sensitivity a produces an output voltage Vout2 as given by Equation (3) below (the temperature of the sensor 1 at this time is assumed to be T0). Subtracting Equation (3) from Equation (2) at each side thereof yields a difference Vd as given by Equation (4) below.                     Vout1        =                  Vss1          +                                    nkT              q                        ⁢                          ln              ⁡                              (                                                      q                    nkTC                                    ⁢                                      ∫                                                                  L                        ·                        α                                            ⁢                                              ⅆ                        t                                                                                            )                                                                        (        2        )                                          Vout2          =                      Vss1            +                                          nkT0                q                            ⁢              ln              (                              q                                  nkT0                  ·                  C                                            ⁢                              ∫                                                      L0                    ·                    α                                    ⁢                                      ⅆ                    t                                                                                      )                            (        3        )                                Vd        =                                            nkT              q                        ⁢                          ln              ⁡                              (                                                      q                                          nkT                      ·                      C                                                        ⁢                                      ∫                                          L                      ·                                              ⅆ                        t                                                                                            )                                              -                                    nkT0              q                        ⁢                          ln              ⁡                              (                                                      q                                          nkT0                      ·                      C                                                        ⁢                                      ∫                                          L0                      ·                                              ⅆ                        t                                                                                            )                                              +                                    nk              q                        ⁢                          (                              T                -                T0                            )                        ⁢                          ln              ⁡                              (                α                )                                                                        (        4        )            
During image shooting, when the temperature of the sensor 1 equals to T0, the third term of the right side of Equation (4) equals 0, which means that the term that depends on the sensitivity xcex1 of the pixel disappears. Therefore, in this case, by subjecting the image data fed from the individual pixels of the sensor 1 to correction as defined by Equation (4), it is possible to suppress the unevenness in sensitivity among the individual pixels. However, when the temperature of the sensor 1 differs from T0, the term that depends on the sensitivity xcex1 of the pixel remains, and therefore, even if the image data fed from the individual pixels of the sensor 1 is subjected to correction as defined by Equation (4), it is not possible to suppress the unevenness in sensitivity among the individual pixels.
As described above, if the temperature at which the image data is acquired differs from the temperature at which the shading data is acquired, it is not possible to correct properly the unevenness in sensitivity among the pixels on the basis of that shading data. For this reason, the sensor 1 needs to be illuminated with uniform light to acquire appropriate shading data anew on every shooting occasion. Quite inconveniently, this requires either additional provision of a means for irradiating uniform light or manually performing shading with a diffusion cap fitted on the lens.
An object of the present invention is to provide an image-sensing apparatus incorporating a solid-state image-sensing device that permits the unevenness in sensitivity among the individual pixels thereof to be corrected properly on the basis of previously stored shading data irrespective of temperature variation.
To achieve the above object, according to one aspect of the present invention, an image-sensing apparatus is provided with: a solid-state image-sensing device composed of a plurality of pixels each having a characteristic that the output voltage thereof varies natural-logarithmically in accordance with the amount of light received; a detector for detecting the temperature of the solid-state image-sensing device; a storage device for storing first output data as to the output voltages from the individual pixels of the solid-state image-sensing device as acquired by shining uniform light on the solid-state image-sensing device, and for storing first temperature data as to the temperature of the solid-state image-sensing device as detected by the detector during acquisition of the first output data; a calculator for calculating, on the basis of the first temperature data stored in the storage device and second temperature data as to the temperature of the solid-state image-sensing device as detected by the detector during image shooting, the temperature ratio of the second temperature data to the first temperature data; a first corrector for correcting, on the basis of the temperature ratio calculated by the calculator, the first output data stored in the storage device; and a second corrector for correcting, on the basis of the first output data corrected by the first corrector, second output data as to the output voltages from the individual pixels of the solid-state image-sensing device as acquired during image shooting.
According to another aspect of the present invention, an image-sensing apparatus is provided with: a solid-state image-sensing device composed of a plurality of pixels each having a characteristic that the output voltage thereof varies natural-logarithmically in accordance with the amount of light received; a detector for detecting the temperature of the solid-state image-sensing device; a storage device for storing first output data as to the output voltages from the individual pixels of the solid-state image-sensing device as acquired by shining uniform light on the solid-state image-sensing device, and for storing first temperature data as to the temperature of the solid-state image-sensing device as detected by the detector during acquisition of the first output data; a calculator for calculating, on the basis of the first temperature data stored in the storage device and second temperature data as to the temperature of the solid-state image-sensing device as detected by the detector during image shooting, the temperature ratio of the second temperature data to the first temperature data; a first corrector for correcting, on the basis of the temperature ratio calculated by the calculator, second output data as to the output voltages from the individual pixels of the solid-state image-sensing device as acquired during image shooting; and a second corrector for correcting, on the basis of the first output data stored in the storage device, the second output data corrected by the first corrector.
According to still another aspect of the present invention, an image-sensing apparatus is provided with: a solid-state image-sensing device composed of a plurality of pixels each having a characteristic that the output voltage thereof varies natural-logarithmically in accordance with the amount of light received; a calibrator for calibrating, on the basis of calibration data for correcting variations in the output voltages from the individual pixels of the solid-state image-sensing device, output data as to the output voltages from the individual pixels of the solid-state image-sensing device as acquired during image shooting; and a temperature compensator for performing temperature compensation, on the basis of temperatures of the solid-state image-sensing device as detected during acquisition of the calibration data and as detected during image shooting, on either the calibration data or the output data acquired during image shooting.